1. Field of the Invention
This invention relates to a system for examining the tissue of an object to be investigated and is especially directed to in vivo measurement of the sound velocity of the tissue. Through such measurement, the user of the system and method can determine the nature of the tissue and often perform medical or scientific diagnosis.
2. Description of the Prior Systems
This invention is related to the United States patent applications Ser. No. 737,472 of Iinuma, filed on May 24, 1985 and now U.S. Pat. No. 4,653,505; Ser. No. 816,902 of Hirama et al, filed on Jan. 7, 1986, and Ser. No. 875,687 of Okazaki, filed on June 18, 1986, each of which is hereby incorporated by reference. Each of the above applications are commonly owned and the reference to them in this application is not an admission that all or any of the applications constitute prior art which can be applied against this application.
Iinuma discloses a basic system and method for examination of the tissue of an object using an intersecting ultrasonic transmitting beam and receiving beam directed from or toward a phased array of ultrasonic transducer elements. A first sub-array on the phased array directs ultrasonic pulses toward the tissue along the transmitting beam. A distinct second sub-array on the phased array and distant from the first sub-array receives echos of the ultrasonic pulses reflected or scattered by the tissue along the receiving beam. The transducer elements of the first and/or second sub-arrays are excited in different timings to steer one or both of the transmitting and receiving beams so that the transmitting and receiving beams intersect at a cross point in the tissue. The propagation time required for the ultrasonic pulses from the first sub-array to travel to the second sub-array through the cross point is measured. A problem of this system is that it is difficult to obtain the actual steered angle of the transmitting and/or receiving beams because the actual steered angle is a function of the ultrasonic velocity in the tissue. The ultrasonic velocity in the tissue, however, is unknown. In that system the propagation length is calculated from the distance between the first and second sub-arrays, the distance between the adjacent elements, and the delayed time between the activation of the adjacent elements to steer the beam, using known geometric principles and Snell's law. Through that system, the average ultrasonic velocity from the first sub-array to the second sub-array through the cross point is obtained from the propagation length and the measured propagation time.
However, organs like a liver are covered by a fat or muscle layer, in which the ultrasonic velocity varies. So it is difficult to obtain an accurate ultrasonic velocity without in some manner cancelling the deviation caused by the fat and/or muscle layer.
Hirama et al. discloses a system and method for obtaining an accurate ultrasonic velocity by cancelling the effect of the fat and/or muscle layer by using two or three different crossed beams. Two crossed beams are sufficient, in a case where the fat or muscle layer has a uniform thickness. The use of three beams is preferable when the fat or muscle layer has a non-uniform thickness.
Okazaki discloses a system or method for mapping the ultrasonic velocities at the different points in the tissue.
In those systems and methods, a B-mode image is first obtained and frozen on the monitor. After that, an operator adjusts the cross point or the beams on the frozen B-mode image of the monitor.
Although the above inventions represent improvements over the prior art systems and methods of measuring the sound velocity within an internal tissue, they do not overcome all problems in this important diagnostic field. A patient's body is moving while one attempts to measure the sound velocity of his inner tissue. As a result of this movement, a blood vessel or similar object is likely to intrude upon the propagation path of the ultrasonic beams. Such a blood vessel or object will reflect the ultrasonic pulse much stronger than the normal tissue, such as the liver itself, and will cause an error in determining the sound velocity.